Question: Before the previous stop there were 50 people riding on a train. 21 people ended up getting off at the stop. How many people are riding the train now?
Explanation: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $50 - 21$ people on the train. $50 - 21 = 29$ people are on the train.